The invention relates to the central processing unit (CPU) of an electronic computer having three machine levels. The first level is the instruction level, the second level is the microinstruction level, the third level is the nanoinstruction level. In particular the present invention is directed to the hardware structures comprised in the second and third machine levels, which are used during the execution of each microinstruction.
As is known, there are two types of central units. The first of these utilizes a sequence logic matrix for generating the signals which execute the microinstructions. The second type comprises a read only memory (ROM) which utilizes the code of the microinstruction for addressing a nanoprogram adapted to supply the signals which act on the logical groups to carry out the processing operations required by the microinstruction.
The central unit hereinafter described is of the second type. The advantage of nanoprogramming is that of permitting greater flexibility in generating the set of microinstructions. In fact, by changing the contents of the nanomemory (the NROM) the set of microinstructions can be changed without any need for modifying the circuits of the machine.
A central unit of the nanoprogrammed type is known in which each microinstruction addresses a corresponding nanoinstruction. This nanoinstruction is transmitted to a logic unit which divides it into a certain number of groups of bits of predetermined lengths. The first group is fixed throughout the execution of the nanoinstruction and is associated in succession with each of the other groups. The execution of the nanoinstruction is obtained by successively utilizing the pairs of groups of bits which are associated from time to time. It follows, therefore, that the time of execution of a nanoinstruction is fixed and is equal to the sum of the time used to carry out the commands generated by each of these pairs. This has the disadvantage that all the nanoinstructions have a fixed duration which is not subsequently fractionable.